Anaerobic fermentation of effluents and waste aims to reduce organic matter and produce energy in the form of biogas. The methanation of sludge in liquid phase coming from the effluent treatment by urban wastewater purification stations, the methanation of the fermentable portion of household waste and the methanation at the farm of manure and other agricultural waste are mainly known. The methanation of effluents from the agrifood industry in liquid phase with a low concentration of dry matter can also be found. Collective co-digestion units, incorporating the treatment of different substrates with different origins (urban, industrial and farming), have been developed.
Fermentation is continuous (matter is continuously or sequentially loaded and unloaded without emptying the digester) or batch (loading of the matter seeded with already fermented matter, with emptying of the digester and reloading with a new substrate).
The person skilled in the art who operates solid substrate methanation plants encounters the limits of the current systems, which methanate with continuous loading, solely for the purposes of mixing fermenting matter in a variable-geometry tank.
The methods for mechanically mixing the matter, moving blades, worm screws, moving cylinder or mixing by a high-pressure or compressed gas, are known. Mixing can be continuous at low pressure or sequential at high pressure, as presented in document FR 2 794 472, in small-diameter pipes or injectors receiving gas at low flow rates and speeds, because of the dispersion over multiple injectors or injection lines with multiple injection openings. Similarly, a digester with several compartments is known. But the known mixing methods, in at least two compartments, do not let slurry close to shear thresholds flow; and they are not suitable for allowing the matter to be stirred in the entirety of a given volume; and they cannot enable control of the fermentation that makes possible significant degradation of the fermenting matter in short retention times and without significant dilution of the matter.
The gas stirring systems that inject gas from the bottom of tanks require an especially large civil engineering structure to access the gas lines exiting at the bottom of the fermentation tank. The lines for intermittently injecting pressurized gas cannot cope with any reflux of matter, which can block the injectors, in the gas pipes. The tip of the injector is necessarily narrow, generating a narrow upwards gas jet that can have only a limited effect on the stirring of slurry; during gas injections this creates preferential passages in the matter that limit the stirring and turning over thereof, in particular for matter at the bottom of the tank, thus making it difficult to put heavy elements back in suspension, with the risk of these accumulating at the bottom of the fermenter. The insufficient stirring thus makes the flow of matter more difficult with phase separations creating dead zones and different fermentation speeds in the fermenter.
It is also noted that the fermentation phases can be sufficiently controlled by significant dilution effects of the matter to boost its flow which also, because of the nature of the stirring, most frequently mechanical or by gas stirring, results in carrying out fermentations in infinitely mixed fermenters. The retention times of the matter are therefore random. There are significant residual liquid effluents at the end of fermentation. For some types of effluent, the lack of dry matter is a factor limiting fermentation through lack of a bacterial medium. This lack of dry matter requires introducing mineral media to fix the bacterial populations that need a medium.